600.000uF SOZ

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Hi guys!

My SOZ (under construction, but already playing) have 4 capacitors 150.000uF each (2 per channel). I I plug the SOZ directly to the mains, the 16A fuse blows....

So, I connected the amp to a extension chord of My PC (it has some kind of protection, varistors and suppresion capacitor) and works fine!

If someone is getting any trouble on the switching on try one of these things.

I guess their not audiofile approved, however I have seen a review of one of these babies on HIFI-CHOICE

Big cap turnon

What is most likely happening is that the additional resistence of the cord is limiting the peak turn on current. You need to put a soft start circuit with about a 10 ohm resistor in series with the AC feed to the transformer. This resistor can then be bypassed with a relay when the filter caps have charged completely. Your bridge rectifiers will probably not last long if you don't use a soft start circuit.


Can you be more specific in the soft start circuit? I'm not familiar with that kind of devices. Nelson Pass talks about thermistors in the SOZ project also, but I don't know how to use them...

My power chord as several components that I can't identify, but I can see 3 Varistors, one of them is in paralell with the mains.

Coul´dn't be it that limits the inrush? the resistance of the chord with the device mentioned is only 0.3 ohm.

a varistor serves to minimize voltage spikes that get thru to your equipment but still allow extremely high currents in the order of hundreds or thousands of amps to be drawn from the mains .... if you place a high power high current thermistor in series with the mains, it will minimize the current that is drawn @ turn on while causing minimal voltage drop during normal operation.
I would be carefull with using thermistors with big capacitance. In my A75 monoblocks (I have 6 of them) I used 100,000uF per channel with 2 bridges 35A ea. channel. I used thermistor as per original design and still have bridge failure from time to time (usually once a year). Using thermistors is convenient but I think that a power resistor (10 to 20 ohm) with a relay would be more reliable.

Do you really think that the chord resistance is enough to take the inrush? shouldn't it melt or at least become warm?

I believe that in the chord switch must be something like a suppresion capacitor or even thermistor, but I cant figure it out by the shape and the markings...

This remind me that my Audionote Kit one Has also a 0.1u
F Suppresion cap in paralell with the mains before the power transformer, but at 450V the capacitance is a little bit less that 600000uF....

Joined 2002
Be Careful of the Current Limiting Resistor Wattage

If you opt to go with the series resistor switched in and out of the AC input, be sure to consider the wattage of the resistor.

If you use a 10ohm resistor on a 120VAC input (240VAC in Europe), the initial inrush current through the resistor will be 12Amps (24Amps in Europe) and will decrease as the supply caps build voltage. A typical amp with a 40VDC supply and 100kuf of supply capacitance will draw nearly the full 12 amps for the first few seconds. Your amp, with 600kuf of capacitance will draw high current for a much longer period of time.

So what:rolleyes:

This means that 120VAC @ 12Amps requires a 10ohm resistor capable of absorbing 1.44kW (Nearly 3kW in Europe)!:eek: This current is passing through the transformer and the bridge to get to the capacitors. What's even worse is the current coming out of the transformer is multiplied by the secondary-to-primary ratio, so you could be looking at 36Amps of inrush current through the bridge.

This is a lot of power. This is one reason I leave my equipment on 24/7 (I also don't like waiting for everything to warms up).

I use a central power box (I built myself) with 2 - 100ohm @ 200W resistors for current limit of 2.4Amps for 15 second. Then I switch in the straight-through by-pass with 3 poles of a 4 pole relay with each contact rated for 30Amps.

Something to think about.

Good luck
Rodd Yamashita
PedroPO said:
Thanks for the project, but since that I am a ZEN kind of guy I'll try the aleph aproach (thermistors).
Do you mean the thermistor is more like Zen because it is a simpler approach? The relay / resistor option leaves less in the AC power path, I really think it is a better solution. Normally I prefer the simpler solution but I would be uncomfortable leaving a thermistor in the circuit.
You've really gone over the top with the capacitors. How about two power switches, like the tube amps with separate switches for filament and plate supply? You can wire two DPDT switches so that it doesn't matter which you turn on first.
Leaving it on all the time may be a good option if you live in an Antarctic research station and use electrical heating anyway. Anyplace else and you're sure wasting a lot of energy. Fossil fuels, global warming, you know.
Joined 2002
Time Delay Relay

I was lucky enough a number of years back to come across an Allen-Bradley industrial time-delay relay. It has a 120VAC triggered input that I connect to my pre-amp switched outlet. It's a 4-pole relay (30Amps/contact) with a 120VAC coil. The solid state time delay is settable from 0.1sec - to about 5min. It also has this LED that flashes as its timing out. I set-up the box so the LED can be seen through a hole in the front panel (cool). A relay like this today would probably run $200 or more.

I bring the hot leg in to the box directly to the resistors and parallel it to 3 time delayed contacts and one non-delayed contact. Then the output of the resistors parallels to the output of the 3 delayed contacts and feeds 4 AC receptacles for my three power amps. The non-delayed contact provides for a pair of simple switched outlets. I also have a pair of unswitched receptacles.

This way, if I want, I can plug the 10AWG power cord for the box into a typical wall outlet, and never max out the outlet. Of course I'm running A/B amp now. It may be a different situation with a couple of class A amp plugged into the box. You might want to run a separate line from the breaker box.

Hope this helps.

Have Fun,
Rodd Yamashita
Nelson Pass:

What is your oppinion about surge protection?

As you know, I had a (little?) problem with my heatsinks(too small), so while I wait for new ones, I tried to put a 24V 60W lightbulb after the transformer ( that brings down the tension to 11V).

I noticed that the fuse blow disapeared.

Does a light bulb act as a "soft start circuit"?

Pedro Oliveira
Blown bridges

Nelson Pass said:
Backing up a bit, you shouldn't be blowing those
bridges as described. Do you have heat sinking for

They mounted on the same sink as output devices. 100,000uF per rail, separate bridge per rail. Heatsink temp. never excede 50 deg. C. The bridges blow at the moment I switch the amp on. Usually once a year. Thermistor is KC004L from Digi-Key, resistance about 7.5 ohm when cold. Happened both to A75 and ZEN.

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